The present invention relates to a pumping system, and in particular to a system for metering and pumping a plurality of fluids.
The accurate metering of a number of fluids simultaneously is necessary in many applications, such as the development of x-rays, development of color film, and so forth. The relative volumes of the different fluids vary from fractions of a cc, i.e., only two or three drops, to nearly 100 cc's. The fluids must be simultaneously metered and then mixed to perform their processing function, and difficulty has been encountered in developing an automated system which can accurately meter the different volumes of fluids. The most critical problem in this area has been the development of a valve or valve system which can accurately meter very small volumes of the fluid as well as accommodating the larger volumes.
The present invention provides a system for metering and pumping a plurality of fluids having wide volumetric differences. The system includes a pumping stage which has a plurality of piston pumps extending from a substantially rigid support to a rack hinged at one end and having an opposite free end. Each of the pumps is associated with one of the respective fluids, and has an expansion cycle for drawing a pre-selected amount of fluid into the pump and a contraction cycle for pumping the fluid. The amount of fluid drawn into each of the pumps during the expansion cycle depends on the distance of the different pumps from the hinged end of the rack, and the pumps are operated by reciprocating the free end of the rack.
The present invention also includes a valve stage having a valve body with a plurality of sets of externally opening ports communicating with a central passageway. Each set includes an inlet port connected to a fluid supply, an outlet port generally opposed to the inlet port, and a common port connected to one of the respective pumps. A valve spool is rotatably mounted within the passageway and has a flat portion corresponding to the inlet and outlet ports of each set and a circumferential indentation corresponding to the common port of each set and integral to the flat portion corresponding therewith.
A power supply is provided which both reciprocates the free end of the rack and operates the valve stage. A timing belt connects the valve spool to the output shaft of the power supply, and an arm eccentrically mounted to the output shaft is connected to the free end of the rack so that operation of the rack and the valve stage is synchronized.
The present invention provides a positively acting mechanical valve rather than passive check valves which are normally found in the prior art. Such check valves are reasonably accurate for relatively large volumes of fluid, greater than a few cc's, but are often inaccurate for fractions of a cc. Such small volumes consist of only a few drops of fluid, and air bubbles can be formed between the drops which destroys the accuracy of the system. Attempts have been made to have two check valves for greater accuracy, but this results in a lower closing pressure exerted on each of the check valves and thus decreases the reliability of the closure.
The mechanical valve arrangement of the present invention allows for the use of a common line leading to the pump, rather than separate feed and return lines as used previously. Besides simplifying the overall system, less fluid remains in the lines so that greater accuracy can be attained in the metering of the fluids, particularly in regard to small volumes. The present invention provides a relatively simple valve system which is still highly accurate, and is thought to be much more accurate than devices found in the prior art.
The apparatus of the present invention allows for a great deal of flexibility in not only the range of volumes metered, but also in the selection of the volumes of each fluid. In a preferred embodiment of the present invention, the pumps are movable along the rack so that their distance from the hinged end of the rack is variable, resulting in a variation in the amount of fluid metered by the respective pumps. Pumps of different sizes can be employed to extend the range of the metering capacity of the apparatus. Also, the speed of the reciprocation of the rack can be varied to alter the volumetric flow of the fluids in unison without changing their relative proportions. The valve stage is operated in synchronization with the rack so that such variation in speed is automatically reflected in the operation of the valve stage.
The novel features which are characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanied drawings which a preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.